Internal combustion engines are well known and widely used for propulsive power, electric power generation, gas compression, liquid transfer, in various industrial applications, for example. In a conventional four-cycle or two-cycle engine operating scheme, a mixture of fuel and air is combusted within an engine cylinder to produce a rapid pressure rise and induce linear travel of a piston, ultimately rotating a crankshaft to provide torque for various purposes. Spark-ignited engines typically employ a liquid petroleum distillate fuel such as gasoline or various gaseous fuels in the nature of natural gas, methane, propane, and various mixtures such as biogas, landfill gas, and mine gas. Compression-ignition engines typically utilize fuels such as distillate diesel fuel, biodiesel, and still other liquid fuels. The manner in which various components of an engine such as valves, pumps, cams, actuators, and still others are constructed and operated can depend to a significant degree upon the type of fuel that is expected to be utilized. Likewise, typical or optimal operating conditions and suitability to particular duty cycles can be closely associated with fuel type.
For example, spark-ignited engines commonly operate at relatively lower cylinder pressures with ignition timing determined by a timing of production of a spark. Compression ignition engines, in contrast, tend to operate at relatively higher cylinder pressures to enable autoignition, with a timing of ignition generally dependent upon when autoignition thresholds are obtained. In-cylinder temperatures and pressures tend to be higher in compression ignition engines than in spark-ignited engines. The manner by which engine emissions are treated also varies across different engine types and fueling platforms. In recent years, there has been significant interest in development of engines and operating strategies that are flexible with regard to fuel utilization. Fuel prices can be fairly dynamic. Moreover, certain fuels that have realized relatively increased abundance in recent years, such as natural gas, can have desirable combustion or emissions properties which are sought to be exploited.
One type of engine design that allows for operation with different fuel types combines diesel fuel and natural gas. Diesel alone is relatively easy to compression ignite, but can produce undesired emissions. When natural gas is used as a fuel in a diesel engine, without modification the mixture of natural gas and air would likely fail to ignite, knock, or have combustion stability problems. Engineers have developed various strategies for predominantly burning natural gas while using a relatively smaller amount of diesel fuel as a so-called pilot fuel. As the mixture of air, natural gas, and diesel pilot fuel is compressed, the diesel pilot fuel can ignite, which in turn ignites the natural gas. Such designs can offer combined advantages of both fuel types. One example of such an engine is known from U.S. Pat. No. 6,032,617 to Willi et al.